Is CaMKII friend or foe for cell apoptosis in eye?: A narrative review

Ca2+/calmodulin-dependent protein kinase II (CaMKII) controls cell proliferation, differentiation, apoptosis, and other biological processes that have an essential role in eye diseases. However, it seems that previous studies have generated conflicting conclusions about the effect of CaMKII on cell apoptosis. In this review, we explore the positive and potentially deleterious effects of CaMKII on eye cell apoptosis. We can safely conclude that the early elevation of CaMKII could be viewed as a promoter of cell apoptosis. Overexpression of CaMKII by transfection or pretreatment with drugs helped combat cell apoptosis.


Introduction
Calcium ion (Ca 2+ ) regulates an array of cellular processes as a highly versatile intracellular signal. [1,2]Ca 2+ /calmodulin-dependent protein kinase II (CaMKII) controls cell proliferation, differentiation, apoptosis, and other biological processes that have an essential role in eye diseases. [2,3]Recently, Bo Chen team [2] published a study in the journal Cell, which concluded that CaMKII is a key regulator of retinal ganglion cell (RGC) survival in normal and diseased retinas, including excitotoxic injury to RGC somas, optic nerve injury to RGC axons, and glaucoma.These results were amazing and made us very interested in the potential use of CaMKII in ophthalmic diseases.6][7] For example, CaMKII may induce neuronal apoptosis under some conditions. [6,7]Whereas under other conditions, CaMKII inhibition protects RGCs from excitotoxic cell apoptosis. [4,5]In this review, we explore the positive and potentially deleterious effects of CaMKII on eye cell apoptosis.

The subtypes and structure of CaMKII
CaMKII has 4 isoforms (α, β, γ, and δ) identified in human tissues. [8]These isoforms are expressed differently in tissues.In eye, α and β are highly expressed in retina nerve cells. [2,9]][12][13][14][15] As shown in Figure 1, each isoform of CaMKII consists of a catalytic domain, an inhibitory domain, a Camodulin (CaM) binding domain, and an association domain.[18] All isoforms had 89% to 93% similar amino acid sequences in their catalytic and autoregulatory domains. [8]The catalytic domain contains adenosine triphosphate (ATP) binding sites and other substrate anchorins.In the inhibitory domain and CaM binding domain, they both have autophosphorylation sites that are in different positions.In the inhibitory domain, the autophosphorylation site of the α isoform was Thr286, and the β, γ, and δ were Thr287.In the CaM binding domain, the autophosphorylation site of the α isoform was Thr305/306, and the β, γ, and δ were Thr306/307.[18] When Ca 2+ binds to the CaM binding domain, the autophosphorylation site of the inhibitory domain is subject to autophosphorylation.And then the catalytic domain releases ATP and activates CaMKII. [17]1]

CaMKII and cell apoptosis
Apoptosis, also known as programmed cell death, is involved in the occurrence, development, and extinction of many eye diseases.RGC apoptosis in glaucoma leads to irreversible vision loss.Photoreceptor cell apoptosis occurs after retinal detachment, and its visual function is difficult to recover even after surgical reduction.Retinitis pigmentosa leads to the death of photoreceptor cells due to gene mutations activating the apoptotic mechanism. [22]CaMKII was involved in apoptosis induced by various factors, such as high glucose (HG), oxidative stress, excitotoxicity, and ocular hypertension.CaMKII regulates apoptosis through endoplasmic reticulum protein phosphorylation, Fas receptor activation, and mitochondrial apoptosis. [23,24]

Apoptosis induced by high glucose
In macaque choroid-retinal endothelial cells (RF/6A), CaMKII activation was induced by HG and then led to the activation of both Fas-dependent and mitochondria-dependent apoptosis pathways. [24]Leukemia inhibitory factor (LIF) significantly inhibited HG-induced human retinal endothelial cells (HRECs) apoptosis by down-regulating the CaMKII/CREB signal pathway. [25]Autocamtide-2-related inhibitory peptide (AIP) as a specific inhibitor of CaMKII inhibited the apoptosis of pericytes in the diabetic retina. [26]Curcumin attenuated HG-induced retinal neuron apoptosis by downregulating CaMKII. [27]So far, there have been few studies on the role of CaMKII in apoptosis induced by HG (as described in Table 1).And the conclusions were consistent: CaMKII activation promoted apoptosis in the HG environment, and inhibition of CaMKII activity by an inhibitor or drug could reduce apoptosis.

Apoptosis induced by oxidative stress
Overproduction of reactive oxygen species is induced by Ultraviolet B (UVB) and blue light.CaMKII inhibitor KN93 blocked cell death, which was mediated by apoptosis-inducing factor (AIF) caused by blue light. [28]In human lens epithelial cells (HLECs), inhibition of cartilage acidic protein 1 (CRTAC1) expression resulted in downregulation of CaMKII expression and eventually reduced apoptosis induced by UVB irradiation. [29]Glutamate up-regulated cell apoptosis through excessive reactive oxygen species generation.In the differentiated Y-79 cells, CaMKII was activated in the glutamate-induced cell injury.Puerarin can dose-dependently attenuate cell apoptosis through the inactivation of CaMKII. [30]In glutamate-treated cells, CaMKIIalpha (B) increases transiently in the early phase.AIP protected RGCs from glutamate-mediated cell death through the inactivation of caspase-3. [31]Interestingly, for RGCs in stressful environments, basal levels of CaMKII can constantly afford them protection. [32,33]In RGCs with knocked-down levels of CaMKIIalphaB by specific siRNA, cell apoptosis was increased by glutamate.Overexpression of CaMKIIαB by pIRES-hyg3/αB enhanced RGC-5 cell survival against glutamate treatment. [32]owever, some studies also found that activation of CaMKII can promote cell survival in a stressful environment.Paeoniflorin (PF) up-regulated the level of p-CaMKII and then triggered AMPK activation against apoptosis induced by oxidative stress in human retinal pigment epithelial (ARPE-19) cells. [34]Combined with Table 2, it can be found that in an oxidative stress environment induced by various factors, the effect of CaMKII activation or inhibition on cell apoptosis was not completely consistent.Under the effects of pathological environments such as blue light, UVB, and glutamate, increased CaMKII activation promoted apoptosis.Cell apoptosis was decreased with CaMKII inhibitor KN93, AIP, and drugs, or with inhibition of upstream protein expression.However, overexpression of CaMKII by transfection or pretreatment with the drug reduced cell apoptosis in oxidative stress, which was induced by glutamate and atRAL, although the effects of the 2 on p-CaMKII levels were opposite.Correspondingly, cell survival decreased induced by glutamate under the knockdown of the CaMKII premise.As described previously, the basal levels of CaMKII may afford cells some ongoing protection against stress. [32,33]So, we speculate that in oxidative stress environments, there could be an indication of a cause-and-effect relationship between the early elevation of CaMKII and the later detection of cell apoptosis.We could treat the early elevation of CaMKII as a trigger for apoptosis.As the apoptotic genes turn on, CaMKII levels decrease, coincident with cell apoptosis.This allows us to understand why cellular CaMKII activation is increased in the oxidative stress environment, but the overexpression of CaMKII is resistant to oxidative stress damage.Table 1 Effect of CaMKII on apoptosis induced by high glucose.

Excitotoxicity induced apoptosis
In the excitotoxicity model induced by N-methyl-D-aspartate (NMDA), activation of CaMKIIα was essential for protecting RGCs. [2]However, previous studies found that CaMKII activity increased in NMDA-induced neuronal apoptosis.AIP attenuated caspase-3 activation and significantly improved retinal ganglion cell survival. [4,5]In addition, the activity of CaMKII is not immutable in NMDA-induced apoptosis (as described in Table 3).After intravitreal injection of NMDA, the activity of CaMKII was elevated for short time periods, such as 30 minutes or 2 hours.But CaMKII activity decreased after 24 hours. [33]Moreover, CaMKII activity changes differently under different concentrations of NMDA.For example, after 2 hours of intravitreal injection of 20 mM NMDA, CaMKII activity is down-regulated, [2] while under 4 mM NMDA, CaMKII activity is up-regulated. [5]It can be seen that the change in CaMKII activity was closely related to the intensity and action time of external adverse factors.There are multiple genes, including apoptosis-associated genes involved in excitotoxicity apoptosis, which can be regulated by AIP. [35]In pathological settings, AIP contributes to inhibiting cell apoptosis.However, in the absence of external adverse factors, the intravitreal injection of AIP resulted in a partial loss of RGC activity. [2]So, it reasonable to think that AIP inhibits cell apoptosis by regulating the expression of apoptosis-associated genes, not only CaMKII.Therefore, we cannot make the conclusion lightly that there were conflicting results about the role of CaMKII in cell survival.

Apoptosis due to ocular hypertension
In the chronic intraocular pressure elevation rat model, brain-derived neurotrophic factor (BDNF) can induce beneficial synaptic changes.Expression of CaMKII, as a key molecule involved in synaptic changes, was upregulated after BDNF injection. [36]urther, RGCs degenerate in glaucoma models, which include elevated intraocular pressure or genetic deficiency.And reactivation of CaMKII through intravitreal AAV-CaMKIIα injection protects RGCs. [2]In terms of the above findings (as described in Table 4), activation of CaMKII can help reduce cell apoptosis.Unfortunately, there are few studies on CaMKII in glaucoma at present, and more evidence is needed to support this view.

Conclusions
In summary, current evidence shows that CaMKII plays an important role in cell apoptosis.The change in CaMKII activity was closely related to the stimulus intensity and action time of external adverse factors.The early elevation of CaMKII could be viewed as a promoter of cell apoptosis.Overexpression of CaMKII by transfection or pretreatment with drugs helped combat cell apoptosis.
Table 4 Effect of CaMKII on apoptosis induced by ocular hypertension.

Authors and PMID Model (Cell/Animal) means of intervention signal pathway Effect of CaMKII on apoptosis
The eye disease involved Hae-Young Lopilly Park et al [36] ; PMID: 31142572 Sprague-Dawley rats cauterization surgery (three episcleral veins were cauterized using a standard technique) IOP elevation BDNF (brain-derived neurotrophic factor) CaMKII-CREB Expression of CaMKII were upregulated after BDNF injection which induce beneficial synaptic changes in glaucoma Glaucoma Xinzheng Guo et al [2] ; PMID: 34297923 C57BL/6 mice was injected magnetic microbeads into the anterior chamber to occlude aqueous outflow.Mice deficient in the glutamate transporter Glast (GLAST¯/¯), a model of normal tension glaucoma.

Table 2
Effect of CaMKII on apoptosis induced by oxidative stress.

Table 3
Effect of CaMKII on apoptosis induced by excitotoxicity.